1. Field of the Invention
The present invention relates to a light emission control circuit, light emission control method, flat illuminating device, and liquid crystal display device equipped with the flat illuminating device and more particularly to the light emission control circuit, light emission control method, flat illuminating device, and liquid crystal display device equipped with the flat illuminating device, which are configured to control driving of a light source made up of a light emitting device such as an LED (light-emitting diode).
2. Description of the Related Art
Conventionally, a display device using a CRT (Cathode Ray Tube) has been used for displaying images, for example, in personal computers, television sets, or a like, however, in recent years, instead of such a display device, a liquid crystal display (LCD) device has become more commonly used. Since a liquid crystal panel is non-luminous, a backlight device is placed on a rear side of the liquid crystal panel and images are displayed by changing optical transmittance of the liquid crystal panel.
As a light source for the liquid crystal display device, from a viewpoint of considerations for environmental problems, mercury (mercury vapor) cannot be used and, therefore, in addition to the CRT, a light emitting device such as an LED is employed. As a result, by using, e.g., red LED(s), green LED(s), or blue LED(s), not only luminance but also chromaticity can be adjusted. That is, it is possible to widen a range of color reproduction (that is, chromaticity region).
The light emission intensity of an LED changes. Therefore, technology has been proposed in which, when driving of a plurality of LEDs is requested, the plurality of LEDs is connected in series to one another and an amount of currents flowing through each of the LEDs is made equal. Incidentally, in the case of the LEDs, a forward voltage to be applied has to be changed so as to correspond to a current to be supplied and, in order to increase a current value, it is necessary that the forward voltage is made higher.
As shown in FIG. 10, a related technology is disclosed in which, in a boost-type DC/DC (Direct Current/Direct Current) converter circuit 101, a plurality of LEDs 102a, 102a, . . . making up an LED group 102 is connected serially to one another, a resistor 103 is connected to a cathode side of the LED group 102 and a control circuit 104 is configured to switch ON or OFF a semiconductor switch 105 so that a voltage across the resistor 103 is made equal to a reference voltage and so that an output voltage is stabilized and a predetermined constant current is supplied to the LED group 102 (for example, see Japanese Patent Reference 1 (Japanese Patent Application Laid-open No. 2002-244103) or a like).
In more detail, the DC/DC converter circuit 101 is made up of the control circuit 104, an inductor 107 connected to a positive terminal of a DC power source 106, a capacitor 108 connected in parallel to the DC power source 106, a diode 109, the semiconductor switch 105 connected in parallel to the DC power source 106 and the inductor 107, and a capacitor 111 connected in parallel to the diode 109 and the semiconductor switch 105.
Moreover, the DC/DC converter circuit 101 is turned ON/OFF at a specified duty ratio and a voltage is outputted at a boosted level relative to a power voltage Va. However, the conventional technology presents a problem in that, in the case of using, as a light source, three kinds of LEDs (red LED, green LED, and blue LED), three sets of circuits to supply a constant current, which causes the configurations of a power source circuit to be made large-scaled, thus causing a rise in costs. In the case of the backlight device of a large-sized liquid crystal display device and a large number of LEDs is used, if a voltage boosting circuit or control circuit for every color is to be mounted, it causes the circuit to be large-scaled, which leads to an increase in costs.
Also, another related technology is disclosed in which an LED display device separately has, as shown in FIG. 11, a power source 202 to drive a light emission device group 201 made of LEDs and a power source 204 to drive a control circuit 203 and, further, a plurality of (a plurality of pairs of) LEDs 201a and 201b connected, in parallel to the power source 202 (for example, see Patent Reference 2 (Japanese Utility Model Laid-open No. Hei 06-002391) or a like).
Each of the LEDs 201a and each of the LEDs 201b make up a pair. An anode side of the LEDs 201b are connected in parallel to each switching element 205 and are time-division driven. Cathode terminals of the LEDs 201a are connected to one another and cathode terminals of the LEDs 201a and LED 201b are connected to a constant current circuit 206 being driven according to a display signal. That is, to each of the switching elements 205 are connected pairs of LEDs 201a and LEDs 201b and to the constant circuit 206 are connected two sets of the LED group.
However, the conventional technology has also problems. That is, when this conventional technology is applied to a backlight device, a forward voltage changes depending on a current fed to each of the LEDs 201a and, as a result, when there is a difference between currents If1 and If2 flowing from two sets of the LED group, current consumption wastefully increases.
For example, as shown in FIG. 12, a voltage VL applied by the power source 202 is the sum of voltages V1 (Va1, Vb1), V2 (Va2, Vb2), and V3 (va3, Vb3) to be applied respectively to the switching element 205, the LED 201a (LED 201b) and the constant current circuit 206. However, the voltage to be applied to the switching element 205 varies greatly depending on whether the current |If1|(|If2|)to be fed to the LED 201a (LED 201b) is comparatively large (that is, when VL=Va1+Va2+Va3) or whether the current |If1|(|If2|) is comparatively small (that is, when VL=Vb1+Vb2+Vb3).
That is, even when the current |If1|(|If2|) is comparatively small, a voltage Vb1 to be applied to the switching element 205 becomes large by an amount of a voltage Vb2 [increment ΔV (=Vb1−Va1)] to be applied to the LED 201a (LED 201b) and power is wastefully consumed in the switching element 205.
Further, still another related technology is disclosed in which a constant current circuit is connected to a cathode terminal of each of LEDs and, when a power voltage is lowered, an LED having the highest forward voltage, out of LEDs being in operation, is detected based on a voltage of the constant current circuit and the power voltage is boosted up to a predetermined voltage so as to correspond to the forward voltage to supply the boosted voltage to each of the LEDs (for example, see Patent Reference 3 (Japanese Patent Application No. 2006-066776) or a like).
Further, still another related technology is disclosed in which a plurality of light emitting units each made up of LEDs of three colors and a switch is connected to each of the LEDs and a constant voltage circuit is mounted on every light emitting device and each LED is driven by either of a simultaneous light emitting method or a field sequential driving method to mix colors (for example, see Patent Reference 4 (Japanese Patent Application Laid-open No. 2006-278252) or a like).
Incidentally, still another related technology is disclosed in which, in a display panel using, as a light emitting device, an organic EL (electroluminescence) device, by letting a feeble current flow through the organic EL, a forward voltage appearing at this point of time and, based on the forward voltage, a forward voltage appearing when a predetermined light emission driving current is supplied to the organic EL is estimated to set an output voltage of a power source circuit (for example, see Patent Reference 5 (Japanese Patent Application Laid-open No. 2006-284859) or a like). However, in the Patent References 3, 4, and 5 a constant current circuit is provided, for example, in every LED or in every light emitting unit (light emitting device), which causes an increase in power consumption.
Therefore, the problem to be solved is that the above conventional technology causes a power source circuit to become large-scaled, which leads to an increase in costs and in power consumption.